In accordance with recent spread of cordless equipment, there are rapidly increasing demands for secondary batteries. In particular, a nickel-metal hydride storage battery, which has a higher energy density than a nickel-cadmium storage battery and a lead storage battery, is regarded as a noticeable power source for an electric vehicle and the like.
A nickel-metal hydride storage battery uses a hydrogen-absorbing alloy electrode as a negative electrode and a nickel electrode as a positive electrode, and is designed to have a positive electrode capacity smaller than a negative electrode capacity. Thus, an oxygen gas generated from the positive electrode in over-charge is absorbed by the negative electrode so that the pressure within the battery can be prevented from increasing. Accordingly, the pressure within the battery can be prevented from increasing even when the battery is over-charged as far as the oxygen gas is rapidly absorbed by the negative electrode.
As the hydrogen-absorbing alloy electrode, any of various electrodes including a pasted electrode, a foamed metal electrode and a sintered electrode can be used. A pasted electrode is fabricated by shaping a punching metal coated with a hydrogen-absorbing alloy and a binder. A foamed metal electrode is fabricated by shaping a spongy conductive substrate coated with a hydrogen-absorbing alloy and a binder. A sintered electrode is fabricated by sintering a conductive substrate, such as a punching metal, coated with a hydrogen-absorbing alloy and a binder. The pasted electrode and the foamed metal electrode, which are not sintered, are poor at oxygen absorbing power because the binder prevents the hydrogen-absorbing alloy from absorbing oxygen.
In contrast, in the sintered electrode, almost all the binder is lost through decomposition during the sintering, and hence, the binder never prevents the hydrogen-absorbing alloy from absorbing oxygen. However, when an oxygen-containing binder (a binder including an oxygen atom in its molecule) or a binder aqueous solution (a water soluble binder dissolved in water) is used as a binder, which includes oxygen which oxidizes the alloy, the hydrogen-absorbing alloy is unavoidably oxidized during the sintering. Accordingly, development of a sintered hydrogen-absorbing alloy electrode with large oxygen absorbing power is significant for obtaining a highly reliable battery in which increase of the pressure is minimal.
The present invention was devised under these circumstances, and an object is providing a sintered hydrogen-absorbing alloy electrode exhibiting large oxygen absorbing power and a highly reliable nickel-metal hydride storage battery using the same.